1.5Km wireless bridge with 2 cpe510

The antennas are very high, there are no obstacles.

In /etc/config/wireless I have set "txpower '13'"

config wifi-device 'radio0'
        option type 'mac80211'
        option hwmode '11a'
        option path 'platform/ahb/18100000.wmac'
        option country 'IT'
        option distance '1600'
        option htmode 'HT20'
        option legacy_rates '0'
        option txpower '13'
        option channel '44'

Why do I see "Tx-Power: 9 dBm" in Lucy?

try change option country from italia to usa

Much better.
Tx-Power: 21 dBm
Speed 18~24Mb
Signal -77
Noise -95

I will test the connection for a while and let you know.